Thank God someone has raised this issue at last!
It comes up frequently. Turning off chat is the best option unless they're particularly abusive, in which case capture it and report them. There are idiots everywhere... Beat them, and then when they ask for a rematch, reject it, then make an invite with an unrated game and send it.
"..who cares if a guy trash talks if it really hits you hard then you have insercutiy issues, it should be equivalent to being called a pussy by some drunken hobo who obvious has done nothign with his life: "am i really a pussy? omg i am? this homeless man knows my life so well"
"You're not wrong Walter, you're just... an A**HOLE" -Big Lebowski